1. Field of the Invention
The present invention relates to a lubrication system for an outboard motor. More specifically, the present invention is suitably applicable to a scheme for preventing oil accumulated in an oil passage downstream of an oil filter provided in a lubrication system for an outboard motor from flowing back and flowing to the outside of the outboard motor.
2. Description of the Related Art
An engine of an outboard motor includes a lubrication system for circulating lubrication oil in the engine to lubricate sliding surfaces of a crankshaft, a camshaft, and so forth. The lubrication system includes an oil filter for filtering lubrication oil. An oil leak hole is formed in an oil relief valve to return lubrication oil that is accumulated in a portion of an oil passage upstream of the oil filter to an oil pan when the engine is stopped.
JP-A-Hei 9-273406, for example, discloses a scheme for efficiently returning lubrication oil to an oil pan by allowing lubrication oil, which has lubricated bearing portions for a crankshaft and which is sticking to the rotating crankshaft, to smoothly flow to the bottom of a crank chamber. This is achieved by forming vertically communicating oil return holes in crankcase-side bearing components defining journal bearing portions in a crank chamber in the vicinity of a crankcase wall that faces the cylinders.
The conventional lubrication system for an outboard motor can return lubrication oil that is accumulated in the portion of the oil passage upstream of the oil filter to the oil pan. However, the conventional system cannot return lubrication oil that is accumulated in a portion of the oil passage downstream of the oil filter to the oil pan. Thus, the lubrication oil remains accumulated in the portion of the oil passage downstream of the oil filter when the engine is stopped. Therefore, when the oil filter is removed from the conventional lubrication system for an outboard motor for replacement of the oil filter, for example, the lubrication oil accumulated in the portion of the oil passage downstream of the oil filter flows back in the direction of the oil filter, and flows to the outside to soil the outboard motor.
According to the scheme disclosed in JP-A-Hei 9-273406, lubrication oil after lubricating the bearing portions for the crankshaft flows via the oil return holes to the bottom of the crank chamber. However, lubrication oil before lubricating the bearing portions for the crankshaft remains accumulated in the downstream portion of the oil passage when the engine is stopped. Therefore, when the oil filter is removed from the lubrication system to which the scheme disclosed in JP-A-Hei 9-273406 is applied, the lubrication oil accumulated in the oil passage downstream of the oil filter flows back in the direction of the oil filter, and flows to the outside so as to soil the outboard motor.